[Fix #6] Bootstrap CSS updated for `blog/index` view template.

To make this happen, of course, several other bits had to as well:
* `ApplicationHelper` now includes new `AppwideFlashes` module, which implements a `#show_appwide_flashes` method — for showing flash messages as part of the layout template;
* `application.css` now requires Bootstrap's CSS (and no longer includes boilerplate-intro comments);
* The `blog/blog/index` view now includes Bootstrap class names;
* The application layout template also now includes Bootstrap style names and proper structuring;
* `Gemfile` uses the Github master repo to avoid a bug in the latest packaged Gem file; this will be reverted when an appropriate update is released;
* Specs, specs, specs. :)

RSpec: 25 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 11 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2